A typical industrial robot is previously known from U.S. Pat. No. 3,909,600 and U.S. Pat. No. 3,920,972. Such a robot has a stand which is rotatably arranged on a foot and which supports a first robot arm which is rotatable in relation to the stand. In the outer end of this arm, and rotatable in relation thereto, a second robot arm is arranged. In its outer end this second arm supports a hand which is provided with a tool attachment and which is rotatable in two or three degrees of freedom in relation to the other arm. The robot is provided with control equipment for controlling the position and orientation of the robot hand. For each one of the above-mentioned axes of rotation, servo equipment comprising a driving motor and a position transducer is provided, the position transducer delivering a signal which is a measure of the angle of rotation of the current axis in relation to a reference position. To the servo system of each axis there is supplied a desired value for the angle of rotation of the axis, and the driving motor of the axis causes the robot to move in the current axis until the angular position indicated by the position transducer of the axis corresponds to the desired value supplied to the servo system.
For the position and orientation of the robot hand to correspond to the desired position and orientation, the mechanical structure of the robot and the data which describe it must be known with a high accuracy. This means that it is not sufficient for the nominal robot model to be known but also the individual deviations from the nominal model must be known. These deviations may be: variations in arm lengths, deviations in the orientations of the axes of rotation of the joints, and lateral displacements (offset) of the axes. These deviations arise in the manufacture of the different mechanical components and in the assembly of these. To this is also to be added that the angle indicated by the position transducer of an axis must with great accuracy correspond to the actual angle of rotation of that mechanical part of the robot which is controlled with the aid of the axis in question.
Because of difficulties in carrying out the calibration in a manner which is economical and adapted to production, the currently normal method is only to consider the nominal structure of the robot for describing the geometry of the robot.
For determining the relationship between the position transducer signals of the robot axes and the actual angles of rotation of the arms of the robot, different forms of calibration methods are used.
In one such calibration method, the robot is caused to assume such a position that the actual angles of rotation in the different axes are known, whereupon the angles of rotation indicated by the position transducers are compared with the actual angles. The position transducers may thereafter be adjusted such that the angles indicated thereby correspond to the actual angles. Alternatively, the deviations between the indicated angles and the actual angles may be stored and then be used during operation for correction of the output signals from the position transducers.
According to a previously known calibration method, the different parts of the robot are set at predetermined initial positions with the aid of a spirit-level which is mounted on accurately finished projections, provided for that purpose, on the different parts of the robot. In the initial position, for example, the above-mentioned first arm may be vertical, the second arm and the hand horizontal etc. In this so-called synchronization position, the actual angles in the different axes of the robot are in this way known, the angles indicated by the position transducers may be read off, and for each axis a so-called offset value may be determined which constitutes the difference between the known actual angle and the angle received from the position transducer. However, this method requires mounting of special additional equipment (the water-levels). Further, the different parts of the robot must be designed such that an accurate mounting of the water-levels is made possible, which entails an increased cost of the mechanical parts of the robot. The calibration method must be carried out manually and requires a relatively considerable time expenditure. Further, the method has a limited accuracy.
From Swedish patent application 9000273-4 a calibration method is known in which a parallelepipedic calibration body is used, the position of which in the base coordinate system of the robot must be known. A calibration tool mounted on the robot hand is brought into contact with the calibration body in a number of different robot configurations. Thereafter, the offset values of the position transducer of the robot are calculated on the basis of the kinematic equations of the robot, a model of the relationship between axial position and position transducer signal, the coordinates in the basic system which are known in the calibration positions, and the read and stored position transducer signals. A disadvantage of this method is that the position of the calibration body must be accurately known. It is normally difficult, in a robot installation in practice, to arrange a calibration body such that its side surface has known coordinates without the use of external measuring equipment. Further, according to this method only the offset values of the position transducers are obtained. The method is based on the nominal kinematic model of the robot and does not take into account defects in manufacture in the individual robot.
The final result is that the actual position of a robot does not correspond to the desired one. The deviation may be several millimeters. This means, inter alia, that a robot which is positioned in a production line cannot be replaced, for example after a breakdown, without the robot program being adjusted. This requires a costly downtime for the production line.
The possibility of individually calibrating each robot unit and compensating for defects in manufacture in the robot, and hence increasing the positioning accuracy, has been demonstrated by many scientists. However, the methods have not gained acceptance in practice. To be useful in practice, the method should be possible to use on the shop floor without demands for expensive equipment. The equipment which is required shall be easily movable.